Teva, the world's biggest generic drugmaker, reached the deal with Cephalon as part of a settlement to end a lawsuit over alleged infringement of Cephalon's patents on the drug. This involved cash payments from Cephalon, which Teva later acquired in 2011.

The European Commission said the deal may have pushed up prices of the drug, modafinil.

"The patent settlement agreement between Cephalon and Teva may have caused substantial harm to EU patients and health service budgets," the EU competition enforcer said in a statement.